In the subsea oil and gas industry, it is often desirable to perform certain fluid processing activities on the sea floor. Examples include fluid pumps (both single phase and multiphase) and compressors (both gas compressors and “wet gas” compressors). The subsea pumps and compressors are commonly driven with electric motors, which are supplied by three-phase electrical power via one or more umbilical cables from a surface facility. Especially in cases where the umbilical cable is relatively long, it is desirable to transmit the electrical power at higher voltages through the umbilical cable and use a subsea transformer to step-down to a voltage suitable for use by the subsea electric motors.
The subsea transformer components are often submerged in a transformer oil that is contained within a tank. However, the pass through points of the tank wall, such as for the electrical connections with the supply and load conductors, are potential sources of failure. In order to increase reliability, some subsea transformers have used a “tank-in-a-tank” arrangement that is schematically illustrated in FIG. 10. In some cases a standard transformer tank that is of a type commonly used in surface applications is used as the inner tank, which is then enclosed in a second, outer tank. The tank-in-a-tank designs thus are able to provide a double barrier between the seawater and the active components (windings and core) of the transformer.